Technology for the absolute measurement of optical frequency is essentially required in the age of optical Internet-based information communication. When the wavelength or the frequency, of light used in optical communication can be precisely measured, the wavelength can be finely split to a desired degree. This enables transmission based on Dense Wavelength Division Multiplexing (DWDM), in which multiple wavelengths are divided and separately carried on a single optical fiber. Such a method denotes an optical transmission method in which multiple optical wavelengths are simultaneously transmitted through a single optical fiber. In a typical optical fiber, four-wavelength multiplexing is performed on a system having a transfer rate of 2.5 Gbps per wavelength, and a transfer rate of 10 Gbps is provided, but, in the DWDM, a maximum of about 80 optical wavelengths is simultaneously multiplexed, and thus a transfer rate of about 400 Gbps can be achieved.
Recently, optical frequency measurement technology has met a new turning point because the absolute measurement of optical frequency has become possible, as a femtosecond mode-lock laser for generating a femtosecond pulse ( 1/1015 second), the time duration of which is very short in a time domain, is used.
Optical injection locking is a method of locking the phase of a slave laser, having a wide frequency bandwidth and inferior spectrum characteristics, to the phase of a master laser, having a narrow frequency bandwidth and excellent spectrum characteristics, without requiring an electronic device.
A femtosecond laser is characterized by an optical frequency comb, in which the repetition rate of extremely short pulses in the time domain is identical to the frequency interval in the frequency domain. Recently, with the development of fceo (carrier-envelope-offset frequency) stabilization technology, the absolute measurement of optical frequency using a femtosecond laser becomes possible. This enables the development of a more precise optical frequency standard than a Cesium atomic fountain clock, which is well known to be the most precise clock at the present time, while providing an opportunity to extend the optical frequency standard from the microwave range, such as an atomic clock, up into the optical frequency range without change. However, such technology is disadvantageous in that the frequency of a femtosecond laser comb has been defined, but it is still difficult to select only a single mode and form a single-mode laser having a desired frequency, or to scan the frequency, and in that the optical frequency synthesizers that have been developed to date are very complicated.